1. Field of the Invention
The present invention relates to a positive active material for lithium battery, a lithium battery having such a positive active material, and a method for producing such a positive active material for a lithium battery.
2. Description of the Related Art
In recent years, a lithium ion battery comprising as a negative electrode a carbon-based material and as a positive electrode lithium cobaltate which is a composite oxide having a layered structure has rapidly spread because of its advantage, i.e., high operating voltage and high energy density. On the other hand, lithium cobaltate occurs scarcely and is expensive. Thus, lithium nickelate has been extensively studied as a substitute material.
Lithium nickelate (LiNiO.sub.2) is a layered structure compound having the same crystal structure as lithium cobaltate, which has already been put into practical use. In the crystal structure of lithium nickelate, lithium is arranged to be occluded between layers in edge shear of NiO.sub.6 octahedral. The production of lithium nickelate is normally carried out by a method which comprises mixing Ni(NO.sub.3).sub.2, Ni(OH).sub.2, NiCO.sub.3, NiO, NiOOH or the like as a nickel source and LiOH, LiNO.sub.3, Li.sub.2 CO.sub.3, Li.sub.2 O.sub.2 or the like as a lithium source, and then subjecting the mixture to heat treatment at a temperature of about 600.degree. C. to 900.degree. C. in a stream of oxygen.
However, as reported in "Solid State Ionics", 44, 87, 1990, "Chem. Express", 7, 689, 1992 and 33rd Battery Symposium in Japan, page 21, 1992, lithium nickelate is disadvantageous in that its structure is similar to rock salt structure and thus is liable in the production method to substitution of nickel with lithium ions, thereby giving asymmetric structure that causes capacity decrease.
In this respect, the use of nickel oxyhydroxide as a nickel material has boon attempted. For example, a method has been proposed, which comprises allowing lithium nitrate to act on nickel oxyhydroxide containing cobalt to synthesize lithium nickelate which undergoes uniform charge-discharge reaction as disclosed in Unexamined Japanese Patent Publication No. (kokai) Hei. 8-306360. Further, a method for the application of electrochemically produced lithium nickelate to lithium battery has been proposed, which comprises charging nickel hydroxide electrochemically in a lithium hydroxide solution as disclosed in Unexamined Japanese Patent Publication (kokai) No. Sho. 63-19761.
Moreover, an active material for lithium battery has been proposed, which is made of nickel oxyhydroxide containing cobalt in an amount of from 20% to 75% as disclosed in Unexamined Japanese Patent Publication (kokai) No. Sho. 63-19760.
As mentioned above, various studies have been made of lithium nickelate. Nevertheless, it is still difficult to obtain a sufficient capacity with lithium nickelate. A further difficulty is that lithium nickelate exhibits charge-discharge characteristics that are liable to change by steps, e.g., four stages and shows poor high rate discharge performance.